Valves are perhaps the most important component in any process industry as they are responsible for isolating or varying the flow of various process streams, and to control the rate of reactions in continuous processes. There are many valve designs that are only suitable for a specific duty; for example, butterfly valves and ball valves are primarily designed to isolate a flow but do not offer accurate flow control. A needle valve may be superior in controlling a high pressure fluid flow with great precision but it is designed to control throughput within a narrow operating range. There are other versatile valve designs that are equally capable of flow control and isolation, such as globe valves and diaphragm valves where the amount of valve opening can be designed to correspond linearly with the number of turns.
A valve, regardless of the design, generally consists of a few key components. It typically consists of a valve body containing a dynamic flow control member co-operating with a static valve seat; such flow control member is moved by a stem linked to a handle or actuator. To prevent leakage, seals, gaskets and packings may be used, both static and dynamic. These may be elastomer seals, for example made of PTFE or coated with PTFE for its high chemical resistivity. The valve body may be connected to upstream and downstream pipelines via a flow inlet and a flow outlet, each for example comprising a flange for compressing a gasket for sealing the connection.
In some processes, such as recovery of materials from coal ash, the process fluid comprises a high concentration of irregular coarse solid particles in suspension and/or is generally corrosive. Hence the process equipment and pipelines are required to be manufactured with a high degree of chemical and/or abrasion resistance. For example, a valve manufactured from mild steel will not be suitable for such applications since the surface in contact with the suspension flow may deteriorate rapidly; as such the whole valve will need regular replacing, leading to unscheduled process shut downs. The regions of seating contact between the flow control member and valve seat in particular are most susceptible to attrition damage since the suspension flow speeds up significantly though a narrowed valve opening; a worn out flow control member and or seat does not offer good sealing performance, and hence is incapable of isolating a flow.
Ceramic lined valves are well known, and are used in applications where fluid to be conveyed (whether liquid, powder, suspension or otherwise) is corrosive, abrasive, or both. Some examples may be found in WO0140614A2, U.S. Pat. No. 5,127,430A and U.S. Pat. No. 5,123,439A. Typical materials used in ceramic lined valves include dense ceramics such as alumina and zirconia, with partially stabilized zirconia providing good abrasion/corrosion resistance. Ceramic materials such as alumina and zirconia possess preferable mechanical properties such as excellent hardness and abrasion resistance, as well as being chemically inert and resistant to acidic or caustic materials. However the crystalline structure in ceramic materials is brittle and so a valve body made from a ceramic material or materials alone may fracture easily when it is subjected to shock or severe vibration. Therefore the common practice is to line a metal valve body with ceramic material so that the valve can be made to resist abrasion and corrosion by the process fluid, as well as having the ability to withstand shock and vibration.
Within the valve body, around the operative part of the valve (e.g. for a ball valve, the ball) turbulence can arise, which can lead to cavitation in liquid fluids, and scouring from particulate fluids. For this reason it is desirable to have as few junctions as possible in the ceramic lining, as junctions provide regions of weakness in the linings.
A further junction is where the valve is connected to adjacent pipework. The junction between a valve and the adjacent pipe is conventionally sealed by provision of a gasket between the pipe and the valve [for example of fluoropolymer, e.g. PTFE]. Fluoropolymers are good at resisting corrosive attack but are not so effective at resisting abrasive attack. Lining pipework with dense ceramics such as alumina or zirconia would be both expensive and technically difficult. For corrosive/abrasive fluids silicon carbide and sometimes alumina and zirconia pipe linings are generally used. Although such linings can be made relatively thin, for the most corrosive/abrasive materials relatively thick linings [e.g. 20 mm] are conventionally used. The silicon carbide used for such lining is porous in comparison with alumina or zirconia ceramics, and has less mechanical strength. In addition it is difficult to ensure that the silicon carbide lining is aligned with the end flange of the pipework. This means that if the silicon carbide lining is short of the end flange of the pipework, a gap is present that has to be filled by the gasket between valve and pipework. If the silicon carbide lining is proud of the end flange of the pipework, then it can be damaged when the valve is tightened up to the pipework. In either case, this exposes the gasket material to attack and can in the extreme result in the corrosive/abrasive fluids reaching the metal body of the valve.
DE 102004063270A1 (Cera System Verschliessschutz GmbH) discloses a ceramic lined ball valve that comprises a ceramic ball in cooporation with a ceramic valve seat. However there is no mention of ceramic material protecting the valve inlet/outlet connections. Thus only the vulnerability at the valve opening is addressed, and not at the connection to the upstream and downstream pipework. There are commercial examples manufactured by this manufacturer that comprise linings extending from the valve seat to the flanges for additional protection, however the valve may still be damaged from any corrosive material that seeps through the joints and comes into contact with the metal valve body, for example the at the connection to the valve end flanges. In some examples a PTFE insert is sandwiched between the metal valve body and the ceramic lining to offer additional protection against corrosion, in case a leak in the ceramic lining.
CN203146890U discloses a ball valve with ceramic linings having ceramic end connections that are recessed into metallic pipe connection flanges, in order to protect the metallic flanges e.g. in case of failure or erosion of a flange sealing gasket or the adjacent pipe flange and/or lining. The valve seats are supported on springs so as provide pressure to the valve seat to seal against the valve ball. Additionally the springs allow the components to accommodate thermal expansion in order to remain mobile even at high temperatures. Such design inevitably introduces crevices along the flow path where cavitation may occur or particles may accumulate. Accumulated particles in this crevice may prevent the movement of the seat thereby requiring higher torque to move the valve ball and potentially “jamming” the valve. Furthermore, although the joints between the valve seats and adjacent ceramic lining components are protected by seals, these seals may wear and fail due to movement of the valve seat, allowing corrosive fluids to seep though and corrode the metal valve body. Furthermore stress may concentrate at a right angled area joining the ceramic end connections and the remainder of the lining, making the valve susceptible to damage under shock and vibration.
As a result, a ceramic lined valve that is robust, of simple construction and which provides enhanced protection against corrosion and abrasion is desirable.